Currently py2app is primarily a setuptools command, which leads to code that is hard to test.

As a first step remove this hard dependency:

1. Create an internal API for building apps/plugins
2. Use this API from the setuptools command (that is, the setuptools is basically only used to collect configuration options and pass those to the "real" builder)
3. Add tests that check that py2app configuration options correctly configuratie the real builder (using a mock builder)

This issue will only create a minimal API, an Builder class with a configuration API and a "run" method. Later issues with further untangle the code.